FY 1978

The distribution and composition of suspended particulate matter above the manganese nodule province of the eastern equatorial Pacific Ocean

Baker, E.T., and J.B. Nevins

In Proceedings of the 10th Annual Offshore Technology Conference, Houston, TX, 8–11 May 1978, 751–758 (1978)

Baseline measurements of the spatial and temporal variability of suspended particulate matter (SPM) within a proposed deep-sea mining area were conducted during 1975 and 1976. A total of 36 stations were occupied along three 360-nautical-mile N-S transects centered at 15°N, 126°W; 11°40N,138°25W; and 8°25N, 150°40W. In general, the water column showed typical open-ocean SPM concentrations: 20–100 µg liter in the euphotic zone, <15 µg liter in the intermediate and deep waters, and a slight increase in the bottom waters. Distributions of SPM within the upper 200 m varied both zonally and meridionally, apparently in response to the equatorial current systems. The highest SPM concentrations occurred above the inferred divergence between the North Equatorial Current and the North Equatorial Countercurrent. Temporal changes in the SPM distribution were noted. Seasonal variations were slight, except for changes attributable to migration of the upwelling zone associated with the equatorial current systems. Diurnal variations were measured during 38-hour time series observations at three stations and consisted of 10- to 20-m vertical excursions of the SPM contours that occurred with a period of 4 to 20 hours or less. Elemental chemistry analysis by thin-film x-ray fluorescence indicates that most (>85 percent) of the SPM in the surface, intermediate, and deep waters is of biogenic origin. Increased SPM concentration in the bottom water is a result of an increase in the nonbiogenic (aluminosilicate) fraction by resuspension of local bottom sediments. These baseline measurements provide information concerning the natural variability of SPM in a proposed mining area. Background values of SPM may be used to assess environmental changes caused by deep-sea mining operations. Monitoring of a mining debris plume by optical or gravimetric methods is feasible only while its concentration above the ambient SPM significantly exceeds the natural variability. Chemical analysis of collected SPM, however, should allow detection of even relatively low concentrations of the predominantly nonbiogenic mining debris.